A thermal flow meter that measure a flow rate of gas is configured to include an air flow sensing portion for measuring a flow rate, such that a flow rate of the gas is measured by performing heat transfer between the air flow sensing portion and the gas as a measurement target. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. The thermal flow meter is characterized in that a flow rate of gas such as a mass flow rate can be measured with relatively high accuracy, compared to other types of flow meters.
However, it is desirable to further improve the measurement accuracy of the gas flow rate. For example, in a vehicle where an internal combustion engine is mounted, demands for fuel saving or exhaust gas purification are high. In order to satisfy such demands, it is desirable to measure the intake air amount which is a main parameter of the internal combustion engine with high accuracy. The thermal flow meter that measures the intake air amount guided to the internal combustion engine has a bypass passage that takes a part of the intake air amount and an air flow sensing portion arranged in the bypass passage. The air flow sensing portion measures a state of the measurement target gas flowing through the bypass passage by performing heat transfer with the measurement target gas and outputs an electric signal representing the intake air amount guided to the internal combustion engine. This technique is discussed, for example, in JP 2011-252796 (PTL 1).
However, it is known that pollutant such as dust (for example, sand) or oil contained in the atmosphere is mixed in the intake pipe of a vehicle having an internal combustion engine. For this reason, an air cleaner is provided in the intake pipe of the internal combustion engine, so that most of the pollutants (for example, particles having a relatively large particle diameter such as sand) are removed by the air cleaner. However, for example, minute particles having a particle diameter of 15 μm or smaller may reach the air flow sensing portion through the air cleaner, or pollutants deposited on the air cleaner may reach the air flow sensing portion due to aging of the air cleaner, so that this may degrade measurement accuracy disadvantageously.
In recent years, from the viewpoint of regulatory reinforcement for exhaust gas, fuel efficiency improvement or the like, even in a state in which intake pulsation is generated in the internal combustion engine or in a state in which a pulsation increases and an air flow (backward flow) directed from the internal combustion engine to the air cleaner of the intake pipe is generated, it is desired to measure an air flow rate with high accuracy. In order to measure the air flow rate with high accuracy even in such states of the internal combustion engine such as intake pulsation, in the corresponding technical field, it is desirable to develop a thermal flow meter having a response speed or a direction detection capability that can follow the intake pulsation.
For such a problem, for example, in JP 2003-262144 A (PTL 2) or JP 2006-258675 (PTL 3), there is discussed a technique for solving the aforementioned problem, in which an orifice is formed on a wall surface of the bypass passage to rectify a measurement target fluid flowing through the bypass passage, so that measurement accuracy is improved.
In the device discussed in PTL 2, a base is arranged with an inclination with respect to a flow of the measurement target gas flowing through the bypass passage to hide a sensing portion, and an orifice portion for narrowing a flow of the measurement target gas is formed on a side wall in the vicinity of the base inside the bypass passage. In addition, a gap between the orifice portion and the base is large in the upstream side of the bypass passage and is small in the downstream side.
In the device discussed in PTL 3, an orifice shape is provided in the vicinity of the flow rate detection element of the bypass passage such that forward and backward flows flowing to a bypass passage having a curved portion having a diameter gradually reduced in the upstream are directed to the flow rate detection element, and an apex of the orifice portion is arranged in the downstream side of the flow rate detection element for the forward flow.